1. Field of the Invention
The present invention relates to a digital watermark detection method and apparatus useful in preventing illegal copies of a digital video signal provided via, for example, a recording medium.
2. Description of the Related Art
Due to the prevalence of apparatuses for recording and playing back digital image data, such as a digital video tape recorder (VTR), a digital versatile disk (DVD), and the like, various digital moving images which can be played back by these apparatuses are being provided. Moreover, various digital moving images are distributed via digital television broadcast via the Internet, broadcast satellite, communication satellite, and the like, enabling users to enjoy high-quality digital moving images.
It is easy to form high-quality copies from digital moving images on the digital signal level. Therefore, unless there is some kind of copy protection or copy control applied to the digital moving images, they may be at risk for unrestricted formation of copies. Accordingly, in order to prevent illicit copies of digital moving images or restrict the generation number of copies formed by authorized users, a method to restrict copies by appending information for copy control to each digital moving image and preventing illicit copies or restricting copies using this appended information is being devised.
Digital watermarking is known as a technique for superposing additional information to a digital moving image as such. In digital watermarking, information such as identification information of the copyright owner or user of contents, right information of the copyright owner, use conditions of contents, secret information required upon using the contents, the aforementioned copy control information, or the like (such information will be referred to as watermark information hereinafter) is embedded in contents including audio data, music data, moving image data, still image data, which has been converted into digital data, so as not to be easily perceived. By detecting the embedded watermark information from the contents later as needed, copyright protection, including use control and copy control, can be achieved, and further use of the contents is possible.
Various methods have been proposed for a digital watermarking scheme. As one of them, a method that applies a spread spectrum technique is known. In this method, watermark information is embedded in a digital moving image in the following sequence.
In step E1, an image signal undergoes spread spectrum by being multiplied by a PN (Pseudorandom Noise) sequence.
In step E2, the image signal after spread spectrum undergoes frequency transformation (e.g., DCT).
In step E3, watermark information is embedded in the image signal by changing the values of specific frequency components.
In step E4, the image signal undergoes inverse frequency transformation (e.g., IDCT).
In step E5, the image signal undergoes inversely spread spectrum (the image signal is multiplied by the same PN sequence as in step E1).
Watermark information is detected in the following sequence, from the digital moving image, in which the watermark information has been embedded in the above sequence.
In step D1, the image signal undergoes spread spectrum by being multiplied by a PN (Pseudorandom Noise) sequence (the same PN sequence as in step E1).
In step D2, the image signal after spread spectrum undergoes frequency transformation (e.g., DCT).
In step D3, the embedded watermark information is extracted from the image signal while paying attention to the values of specific frequency components.
Meanwhile, in JP-A 2002-325233 (KOKAI), particularly claim 2 and FIG. 7 disclose a technique to embed a specific frequency component of an embedding target image to the embedding target image after controlling the phase and amplitude of the specific frequency component in accordance with the watermark information, and also a technique to detect the watermark information embedded thereby. Particularly, when detecting the watermark information, the correlation (cross-correlation or phase only correlation) between an input image (watermark embedded image) and the specific frequency component extracted from the input image is utilized in order to estimate the embedded watermark information from the peak of a correlation value.
When digital watermarking is applied to digital productions for the purpose of prevention of illicit use, a characteristic (robustness) that can prevent watermark information from being lost or tampered with due to deliberate attacks which are normally carried out on digital productions must be provided to digital watermarking. As an example of attacks which disrupt watermark information detection for digital images embedded with watermark information, cut-out and scaling (enlargement/reduction) of an image are known.
When a digital image that has suffered such attacks is input, the conventional technique applying spread spectrum recovers synchronization of a PN sequence by executing a process for estimating a PN sequence used in step E1 at the time of embedding upon detection of watermark information. After that, the processes in steps D1 to D3 are executed to extract the embedded watermark information.
However, in order to recover synchronization of the PN sequence, a search must be conducted in which a synchronization recovery process for a plurality of candidates of PN sequences is attempted, and a candidate that has achieved the synchronization recovery properly is adopted. For this purpose, calculation amount and circuit dimension increase. Further, since the watermark information of a digital image having undergone attack is weakened, even if the contents (cut-out, scaling, etc.) of the attack is identified and a corresponding detection is carried out, it is difficult to detect watermark information properly.
When detecting digital watermarking by utilizing cross-correlation between the input image and the extracted specific frequency component, it will be possible to obtain a relatively strong robustness while suppressing the increase in calculation amount and circuit scale against the attack from, such as, cut-out and scaling of an image. However, the peak of the correlation value generated by cross-correlation is not so steep.
Meanwhile, in JP-A2005-252491 (KOKAI), according to a digital watermarking detection technique utilizing a phase only correlation as described particularly in claim 1 and FIG. 1, a steep peak appears in the correlation value since the correlation is taken by fixing the amplitude of each signal of the input image and the extracted specific frequency component. If the peak of the correlation value is steep, it will be easy to estimate the watermark information. However, if the amplitude is fixed, the amplitude error between the signals before and after attack increases. It is preferable that this amplitude error is kept to a minimum in order to increase the robustness against attacks.